There are all manner of theories, but most of the more likely seeming are those that are to do with a lever or fulcrum, as in carrying/moving a load, or in old style farmer’s scales. If it wasn’t properly balanced, the one with short end was disadvantaged. The scale would consist of a notched stick with baskets suspended at each end and a series of known weights used to measure the weight of produce being offered. If the merchant buying the produce shortened one end, he could make the seller appear to be providing less than he was.